Bug Report

A Container[str] is expected to accept any object, even though it is generic over str:

from collections.abc import Container


class StringContainer:
    def __contains__(self, item: str) -> bool:
        return True


container: Container[str] = StringContainer()

$ mypy t.py
t.py:9: error: Incompatible types in assignment (expression has type "StringContainer", variable has type "Container[str]")  [assignment]
t.py:9: note: Following member(s) of "StringContainer" have conflicts:
t.py:9: note:     Expected:
t.py:9: note:         def __contains__(self, object, /) -> bool
t.py:9: note:     Got:
t.py:9: note:         def __contains__(self, str, /) -> bool
Found 1 error in 1 file (checked 1 source file)

This contradicts the error mypy gives when trying to use a Container[str] with any type other than str, if strict mode is enabled:

from collections.abc import Container


container: Container[str] = ()

if 42 in container:
    pass

$ mypy --strict r.py
r.py:6: error: Non-overlapping container check (element type: "int", container item type: "str")  [comparison-overlap]
Found 1 error in 1 file (checked 1 source file)

To Reproduce

See above.

Expected Behavior

I expect to be allowed to define my __contains__ method with the correct argument type.

Actual Behavior

See above.

Your Environment

• Mypy version used: mypy 1.19.1 (compiled: yes)
• Mypy command-line flags: --strict
• Mypy configuration options from mypy.ini (and other config files): N/A
• Python version used: Python 3.14.2 (main, Dec 5 2025, 16:49:16) [Clang 17.0.0 (clang-1700.4.4.1)]